compare: add page (#3632)

pages/common/compare.md

@@ -0,0 +1,12 @@
+# compare
+
+> View the difference between 2 images.
+> More information: <https://imagemagick.org/script/compare.php>.
+
+- Compare 2 images:
+
+`compare {{image1.png}} {{image2.png}} {{diff.png}}`
+
+- Compare 2 images using a custom metric:
+
+`compare -verbose -metric {{PSNR}} {{image1.png}} {{image2.png}} {{diff.png}}`